Add HLRC support for enrich get policy API. (#45970)

Changed the signature of AbstractResponseTestCase#createServerTestInstance(...)
to include the randomly selected xcontent type. This is needed for the
creating a server response instance with a query which is represented as BytesReference.
Maybe this should go into a different change?

This PR also includes HLRC docs for the get policy api.

Relates to #32789

docs/java-rest/high-level/enrich/get_policy.asciidoc

@@ -0,0 +1,32 @@
+--
+:api: enrich-get-policy
+:request: GetPolicyRequest
+:response: GetPolicyResponse
+--
+
+[id="{upid}-{api}"]
+=== Get Policy API
+
+[id="{upid}-{api}-request"]
+==== Request
+
+The Get Policy API allows to retrieve enrich policies by name
+or all policies if no name is provided.
+
+["source","java",subs="attributes,callouts,macros"]
+--------------------------------------------------
+include-tagged::{doc-tests-file}[{api}-request]
+--------------------------------------------------
+
+[id="{upid}-{api}-response"]
+==== Response
+
+The returned +{response}+ includes the requested enrich policy.
+
+["source","java",subs="attributes,callouts,macros"]
+--------------------------------------------------
+include-tagged::{doc-tests-file}[{api}-response]
+--------------------------------------------------
+<1> The actual enrich policy.
+
+include::../execution.asciidoc[]

docs/java-rest/high-level/supported-apis.asciidoc

@@ -604,6 +604,8 @@ The Java High Level REST Client supports the following Enrich APIs:
 
 * <<{upid}-enrich-put-policy>>
 * <<{upid}-enrich-delete-policy>>
+* <<{upid}-enrich-get-policy>>
 
 include::enrich/put_policy.asciidoc[]
 include::enrich/delete_policy.asciidoc[]
+include::enrich/get_policy.asciidoc[]